The invention relates to a drive device for positioning a mirror-glass holder mounted pivotably in a mirror housing of a vehicle rear-view mirror, the drive device having at least one electric motor disposed in a housing consisting of upper and lower parts, the output shaft of the motor being drivingly engaged with a worm gear mounted rotatably and having limited tilting capability in the housing, the worm gear axially moving a nonrotatably supported positioning element, the free end of which is coupled flexibly with the mirror-glass holder, the worm gear being provided in the housing with only one single bearing, which elastically surrounds a coupling part seated at the center of the lower end face of the worm gear and engages positively with the said coupling part by means of an upper annular collar that is snapped into an upper annular slot of the coupling part.
Such a drive device is known from Manzoni U.S. Pat. No. 4,598,600. In this known device, the coupling part is constructed as a ball, which is molded integrally onto the worm-gear underside via a stalk-like nose. The bearing for holding the ball is a snap-in seat in the housing lower part, which consists of an annular wall provided on its upper inside face with a convexly protruding region, which overlaps the ball snapped into the bearing and supported against the bottom thereof, the overlap zone being a spherically annular surface.
In this known drive device, the retaining force of the bearing may not be sufficient in the presence of large pulling forces, as can occur if the positioning element is adjusted manually or encounters its end stop. The reason is that the inside diameter of the convexly protruding region is permitted to be only slightly smaller than the ball diameter of the coupling part, because otherwise its insertion into the bearing or release from the mold of the injection-molding die becomes impossible; the ball is therefore overlapped by the protruding region along a spherically annular surface located immediately above the ball equator. Consequently, the angle that the pulling direction on the one hand forms with the tangent at the point of the overlapped region located closest to the worm-gear axis on the other hand is necessarily predetermined and very acute, which means a small retaining force.